1. Field of the Invention
The present invention relates to speaker systems that employ a quarter wavelength acoustic standing wave transmission line in combination with a speaker. More particularly, loudspeakers with a compact quarter wavelength transmission line and design methods associated with them.
2. Description of the Prior Art
Enclosures have been used with loudspeakers not simply to improve the appearance and decorative appearance of the speaker. Without a speaker enclosure any sound waves emanating from the rear of the speaker that are out of phase with the desired sound waves from the front of the speaker can create interference patterns and can cause cancellation of some frequencies in the desired sound waves. This can be a major problem at lower frequencies where the wavelengths are longest. It has been noted that at the lower frequencies the interference from sound waves from the back of the speaker can affect an entire listening area.
One technique to reduce the interference from the rear of the speaker is providing a transmission line of a selected length coupled to the back of speaker to shift the phase of the output from the rear of the speaker by 90° to 270° to reinforce the output from the front of the speaker. In this technology the speaker is often referred to as a driver as the speaker is said to drive the transmission line. The transmission line is a hollow enclosed path with a length that is a multiple of the quarter wavelength of the primary frequency that interferes with the desired sound frequencies produced by the front of the loudspeaker. Since lower frequencies have the longest wavelengths, low frequency speaker systems that incorporate such a transmission line tend to be larger than other types of woofer and sub-woofer designs. The size of the transmission line and speaker configuration is dictated by the wavelength of the frequency to be compensated. Given the long wavelength of the sound frequencies, the length of the transmission line is generally one-quarter the wavelength of a frequency in the output range of the specific woofer or sub-woofer speaker, thus speaker systems with these transmission lines are typically referred to as quarter wavelength speakers.
Prior art quarter wavelength speakers used a conventional enclosure that was generally a cube or rectangular in shape with the transmission line, in this technology also referred to as a port, internal to such an enclosure requiring the transmission line or port to take the shape of an “elephant's trunk” to provide the necessary length of the port. For those designs the enclosure volume relative to the port volume compromised the performance as the necessary shape of the port contributed to the non-linearity developed since the air resistance of the port (inhale vs exhale) are different. That difference was related to the turbulent flow developed when the speeding air particles collided with sharp corners (enclosure walls and the speaker-port mating area within the enclosure). Another problem of the prior art is the inconvenience of creating the “elephant's trunk” design in a traditional speaker enclosure. Such speakers put the aesthetics of the enclosure having a particular ratio of height, width and depth ahead of the port shape needed to maximize performance of the speaker system thus forcing the port into an “elephant's trunk” shape, or worse. In an effort to control harmonic distortions with the use of an aesthetic enclosure, the port was designed to have a variable cross-section throughout its length to control the level of harmonic distortions that resulted in the requirement to use such enclosures. This only introduced detrimental side effects, such as driver offset that worsened harmonics.
For 56 Hz, given the speed of sound in air being approximately 1130 ft./sec, a single 56 HZ wave has a wavelength of 240 inches, i.e. 20 feet long, thus a quarter wavelength of 56 HZ has a length of 60 inches, i.e. 5 feet. Thus, considering only wavelength, the port would have to be 60 inches long. However, do to other factors than just the wavelength of the selected frequency, a prior art example of a tubular port with a port length of 60 inches the tuning frequency was measured at 39 Hz. Thus, considering only wavelength, the resonance tuning frequency was about 30% lower than expected. It was then determined that the cross sectional area of the port perpendicular to the moving air mass in the port is another factor needed to be considered in the determination of the necessary length of the port for a particular frequency. It was determined that as the diameter of the tubular port is increased there is a larger moving mass of air which lowers the resonance frequency of the tubular port. Another issue with the quarter wavelength designs of the prior art is extremely high harmonic distortions. It was determined that harmonic distortion was related to the cross sectional area of the port and the peak pressure in the port. In addition these harmonics were noted to also be related to the wind velocity in the port, typically wind velocities that exceed the speed of sound by as little as 2% (i.e., 22.5 feet/second).
Two prior art examples of quarter wavelength speakers are described in U.S. Pat. No. 6,425,456 by Jacob George entitled “Hollow Semicircular Curved Loudspeaker Enclosure” issued Jul. 30, 2002 and U.S. Pat. No. 6,634,455 by Yi-Fu Yang entitled “Thin-Wall Multi-Concentric Sleeve Speaker” issued Oct. 21, 2003.
The George patent ('456) illustrates the port in the “elephant's trunk” shape that curls in on itself. In George's design the proximate end of the port has a diameter that is as large as, or larger than, the diameter of the driver with that diameter remaining unchanged for some distance from the driver before turning a corner into a smaller diameter section and then yet smaller diameters in each of the next three turns in the path of the port before opening into a bifurcated output end of the port.
The Yang patent ('455) illustrates in FIGS. 1 and 4 the port as what can be seen to be a cylinder within a cylinder within a cylinder. In his FIG. 1 the output of speaker 12 opens directly into the listening area, while the back of the speaker opens into a first portion of the port that is a large enclosed area that is much larger than the diameter of the speaker and then into a second portion of the port that has a diameter that is considerably larger than the diameter of the speaker with the third and forth portions of the port each having a smaller cross-sectional area than the preceding portion of the port. Each of the transitions from the second to the third, and the third to the fourth portions of the port requires a 180° reversal of the air mass in the port as the air mass transitions from a lower pressure portion of the port to a higher pressure portion of the port each time the path of the port transitions to the next cylinder as the path of the port progresses outward with each of those transitions creating turbulence in the air mass. Additionally, the bottom of the second cylinder is separated from the bottom of the enclosure thus permitting some of the air mass from the fourth portion of the port to collect beneath the second cylinder thus causing yet additional turbulence in the air mass just before exiting the bottom of the enclosure.
Yang, in his FIG. 4, shows an alternative design which is substantially the design of FIG. 1 turned upside-down with the front of the speaker facing the floor with the back of the speaker driving the air mass into the outermost cylinder port and then progressing inward through additional cylinder with a 180° transition between each of them and terminating a large diameter center cylinder that opens to the top into the surrounding atmosphere. This design has similar turbulence problems to those of his FIG. 1 design, plus an added problem. With the top end of the speaker facing the floor and opening to the surrounding atmosphere through the small space below the speaker enclosure, there is a back pressure on the front of the cone of the speaker that is emitting what should be the desired audio signal that is being compensated for by the transmission line. That back pressure is caused by two factors: one is the reflected sound waves from the floor, and the second in the restrictive small space beneath the speaker enclosure that acts as a second transmission line. That second transmission line can change the frequency that is desired to another frequency before being delivered to outer atmosphere in the listening area. Additionally, that back pressure on front of the speaker changes the movement pattern of the speaker cone and thus modifies the air flow through the cylinders of the transmission line which changes the frequency response of the transmission line from the desired response to an unknown response.
The present invention neither looks like the example designs, or any other design that the applicant has seen. Additionally, the present invention was not designed to fit some preselected enclosure, but rather the design of the transmission line defines the enclosure. Thus, as will be realized in the discussion below of the present invention and from the figures, that the present invention is hot a “make fit” design as is the prior art.